A European spacecraft is about to make history’s first landing on a comet.

It has been a long trip for Rosetta. On March 2, 2004, the European Space Agency probe was launched from Kourou, French Guiana, to rendezvous with a comet streaking along at 24,600 mph. Scientists hope that Rosetta will be a codebreaker like its namesake, the ancient tablet that gave scholars the key to deciphering Egyptian hieroglyphics. The new Rosetta, they believe, can unlock clues to the early history of the solar system and to objects that have mystified sky watchers since Babylonian times.

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This animation shows the Rosetta spacecraft’s 12-year journey to reach comet 67P/Churyumov-Gerasimenko. The “cosmic billiard ball,” as the European Space Agency calls Rosetta, took advantage of four gravity assists, three around Earth and one around Mars, to speed up and adjust its course to match the comet’s. On the way, Rosetta flew by two asteroids to test many of its 21 instruments. After a long hibernation as it reached out to nearly Jupiter’s orbit, it will finally reach Churyumov-Gerasimenko this August. Soon after, Rosetta will deploy its lander, Philea, to study the mysterious comet up close, and the trio will travel together around the closest point in its orbit of the sun. (Video: ESA)

Rosetta spent the first seven years of its mission getting into a position where it could eventually intercept its target, Comet 67P/Churyumov-Gerasimenko. After four gravity assists and two asteroid flybys, mission engineers sent the spacecraft coasting on a 957-day hibernation in deep space. On January 20, Rosetta got a wake-up call. Now the bright-eyed spacecraft is making the final approach to its destination for a rendezvous this fall.

A ball of ice and dust about two and a half miles across, Churyumov-Gerasimenko was discovered in 1969 by the Ukrainian astronomers for which it’s named. It’s a short-period comet: It was created in the Kuiper Belt, at the edge of the solar system, and orbits the sun about once every seven years. (Long-period comets have their origins beyond the Kuiper Belt and follow eccentric orbits that can take from hundreds to thousands of years to complete.) Over eons, the gravitational influences of larger bodies have altered Churyumov-Gerasimenko’s orbit, which now extends out near Jupiter. Gerhard Schwehm, who worked with the Rosetta team from 1985 until he retired as mission manager last March, says they had originally targeted another comet, 46P/Wirtanen, for Rosetta’s destination, but in 2003, after a year-long delay due to some issues with the Ariane 5 rocket that would launch the spacecraft, Churyumov-Gerasimenko moved off the back-up list.

After a long sleep to save energy, Rosetta was 45 light-minutes from Earth when an alarm awakened it.
(ESA)

Gerhard Schwehm, who joined the Rosetta team in 1985, says we still have a lot to learn about comets and their unusual activity, such as the dynamics of gas jet eruptions from the nucleus.
(ESA)

Nearly an hour later, the team in Darmstadt, Germany, got the blip they’d been waiting for.
(DLR / CC-BY)

Team members examine a full-size engineering model of Rosetta in a clean room at the European Space Operations Center in Darmstadt.
(ESA / J. Mai)

At the Rosetta “wake-up” event last January 20, the team showed off a model of Philae, the lander. About two miles from the target, Rosetta will drop Philae, and the comet’s weak gravity will gently pull it to the surface. To stay anchored, the lander will punch pitons from its feet and fire a harpoon into the surface.
(ESA / Jurgen Ma)

Scientists’ questions about comets have evolved over the last decade, says Michael A’Hearn, who led another comet chaser mission, NASA’s Deep Impact, to Comet Tempel 1.
(AP / Reed Saxon)

To catch it, the team needed to get Rosetta into the comet’s orbit. As the spacecraft made four orbits around the sun, it flung itself around Earth, then around Mars, and then around Earth twice more, accelerating and adjusting its course along the way. On its route to match the comet’s path, Rosetta also flew by two main belt asteroids, in 2008 and 2010, and field-tested many of its 21 instruments.

In the summer of 2011, Rosetta passed through the main asteroid belt, between the orbits of Mars and Jupiter, and continued out toward Jupiter’s orbit. At that distance from the sun, the spacecraft’s 104-foot-span solar panels were producing less and less energy, so mission controllers put Rosetta into its planned hibernation, shutting down everything but minimal heating for vital components. The spacecraft slept for almost three years. A wake-up alarm went off this past January as Rosetta was inbound on the interception lap, 500 million miles from the sun. At mission control in Darmstadt, Germany, where the event was live-streamed on the Internet, a tall blip appeared on the console, the signal that Rosetta had responded to the alarm. The team erupted into cheers.

As Rosetta approaches the sun, its solar arrays will produce more power, though only gradually, so activating the spacecraft “will be an incremental process,” says Matt Taylor, who chairs the mission’s science team. Rosetta is the only spacecraft to travel such a distance—now more than 3.8 billion miles—using only solar arrays as a power source.

In May, Rosetta will make the first of several maneuvers to set up its rendezvous with the comet. The spacecraft will start its approach by sliding in front of the comet from just inside the comet’s orbit, like a fast-moving car on the inside lane passing a slower one on the outside, then merging to take the lead in the outside lane. Rosetta will then slowly put on the brakes and let the comet catch up to it. The spacecraft will next execute a maneuver to begin imaging the comet’s surface, moving in a triangular path at a distance 60 miles ahead of the comet, says mission manager Fred Jansen, before halving the distance and executing another triangle.

After about two months, Rosetta will draw to within 19 miles, close enough to be captured by the comet’s gravity (only a tiny fraction of Earth’s). Rosetta will wind its orbit closer, while at mission control, scientists will scour the surface maps for a smooth landing site. Rosetta is carrying a 220-pound lander, Philae, named after the Nile River island where archeologists found an obelisk confirming the Rosetta translation. “We fly within three kilometers [1.9 miles] of the surface and drop the lander,” says Jansen.

Philae will come in for a touchdown at only three feet per second—a walking pace—but even a landing this gentle could have enough momentum to cause the lander to rebound; the comet’s gravity might not be enough to hold it down. Philae’s three legs have spring dampeners; when compressed, a piton is punched out that digs into the surface. Almost simultaneously, the lander fires a harpoon for permanent anchoring.